Secondary batteries may be subjected to a variety of different circumstances surrounding their charge and discharge. Quite often a secondary battery may be subjected to rapid charge, and depending on the nature of the charging equipment and its control circuits, a battery may be subjected to overcharge conditions.
Likewise, a secondary battery may be subjected to discharge conditions that range from very gentle to abusive—for example, if the secondary battery is driving an electric motor that stalls.
Under any or all of those conditions, the chemistry of the battery—whether it be a lead acid battery, for example, or an alkaline battery—may be such that the interior volume of the battery may be subjected to varying gas pressure circumstances—both above and below (positive and negative) the ambient pressure within which the battery is operating. Of course, this discussion is directed to such batteries as lead acid batteries, but it is more particularly directed to alkaline batteries, all of which batteries in any event have a reserve of liquid electrolyte within the battery case structure.
Such a battery might be a high performance nickel zinc battery, for example, or even a nickel zinc supercapacator. In any event, any such battery in respect of which the present invention is particularly intended to be used, is typically considered to be a no maintenance battery—one that, apart from charging and the like, may never be checked or may only be checked very seldomly to determine the condition of the battery, the liquid electrolyte in the battery, and so on.
Because such batteries are subjected to variations of internal pressure, particularly pressure build up where the internal gas pressure of the battery rises well above the ambient pressure, such cells and batteries must be provided with means to relieve that internal pressure. Typically, that means the provision of a pressure relief valve, which has a number of functions to perform. They include:
Relief of internal excess gas pressure within the battery;
Prevention of atmospheric oxygen from entering the battery;
Having an operating life of at least equal to that of the battery, so that the pressure relief valve will operate successfully over the life of the battery.
The battery industry has employed a number of designs over the years. Typically, lead acid batteries are provided with a so-called Bunsen Valve, the structure of which is well known to those in the battery art. Such a valve is effectively a check valve, operating in one direction, and having a certain stiffness so as to open only after a gas pressure build up has reached a predetermined pressure.
Alkaline cells, particularly large alkaline cells in batteries of the sort that may be used, for example, in stand by operations and the like, have typically employed a spring loaded relief valve.
However, typically spring loaded relief valves operate at a higher pressure then Bunsen Valves, so that typically they may be activated only when the cell or battery is operating under abusive conditions. A Bunsen Valve will typically operate more frequently, under lower pressure conditions.
However, especially in the conditions or circumstances where a battery is operated in an inverted position—accidentally or deliberately—Bunsen Valves in particular can fail, or they can encounter operating difficulties and problems.
For example, small quantities of electrolyte may accumulate in the Bunsen Valve, when the battery is in an inverted position, or even if the battery is being transported in an inverted position, so that the small quantities of accumulated electrolyte may be expelled when the valve opens. Such circumstances can be damaging to any electronic components that are powered by the battery.
Also, and in any event, the physical presence of electrolyte between the rubber relief valve structure and the sealing surface against which it operates, in a typical Bunsen Valve, may prevent proper function in the future.
In the event of an alkaline electrolyte being present in the region of the pressure relief valve vent structure, that circumstance can be disastrous. Alkaline electrolyte tends to have a low surface tension, and therefore is much more likely to creep or enter the valve mechanism if the battery is in an inverted condition. Also, if there is exposure of the alkaline electrolyte to the atmosphere, then there will be a gradual build of carbonate as a consequence of the reaction of the residual alkaline electrolyte with carbon dioxide in the atmosphere.
Accordingly, particularly if the pressure relief valve is to be operated with alkaline cells, then the structure of the pressure relief valve must be such that no electrolyte may access the valve area under any conditions. To that end, the present invention provides a structure for a pressure relief valve which precludes access of electrolyte—whether it be alkaline or acidic—to the pressure relief valve under any conditions, and whether the battery is an inverted condition or an upright condition. This is accomplished by providing a structure that has a gas flow chamber having a height that extends below the top surface of the battery case structure to a sufficient distance that, if the battery is inverted, the end of the gas flow chamber and a dome that is affixed thereto, as described in greater detail thereafter, will stand above the liquid electrolyte.
Also, when the battery is in an upright orientation, the structure of the dome is such that any liquid electrolyte which attempts to accumulate on the dome will drop off the dome and back into the reservoir.